In recent years, porous filtration membranes such as an ultrafiltration membrane and a microfiltration membrane have been increasingly used in many industrial fields such as: semiconductor industry, e.g., filtration of impurities in a resist solution; water treatment, e.g., drinking water production and water and wastewater treatment; medical field, e.g., blood purification; pharmaceutical field, e.g., virus removal; and food industry. Porous filtration membranes with various pore sizes have been developed. For example, in the semiconductor field, a remarkable improvement in the technique of making finer structures has led to an increasing demand for cleanliness of high-purity chemicals such as photoresist, cleaning liquid, and ultrapure water for use in the production of semiconductors, and this has led to higher demand for the ability of trapping fine particle impurities. Furthermore, in the pharmaceutical field, there has been increasing demand for virus removal and the use of virus removal membranes having a pore size of approximately 10 nm to 50 nm has been increased. In particular, a filtration membrane having a pore size on the order of nm to μm, which has been often used as a porous filtration membrane for trapping such fine particles, is produced using the phase separation of organic polymer solution in many cases. This technique is applicable to many organic polymer compounds and is easy to industrialize. Therefore, at present, this technique is mainly used in industrial production of filtration membranes.
The method for producing a porous filtration membrane can be roughly categorized into non-solvent induced phase separation (NIPS) and thermally induced phase separation (TIPS). In NIPS, a uniform polymer solution undergoes phase separation through the addition of a non-solvent. On the other hand, TIPS is a relatively new method, by which phase separation is induced by cooling a uniform polymer solution, obtained by dissolving a polymer at high temperature, to a temperature equal to or lower than the binodal line that is a boundary between the one phase region and the two phase region, and the structure is fixed by crystallization or glass transition of the polymer.
Conventionally, in many cases, porous filtration membranes have been usually made from polyolefin such as polyethylene or polypropylene, polyvinylidene fluoride, polysulfone, polyether sulfone, polyacrylonitrile, cellulose acetate, or the like. However, polyolefin, polyvinylidene fluoride, polysulfone, polyether sulfone, and the like have problems in that they are highly hydrophobic and thus reduce the flow of water, and that they have properties of adsorbing hydrophobic substances such as proteins and thus readily foul and cause a reduction of water permeability in the case of a liquid containing a large amount of hydrophobic substances. In the fields of pharmaceutical and food industries, there have been serious problems in that the adsorption of proteins leads to loss of useful proteins, change in flavor, and the like. On the other hand, as for hydrophilic impurities, polyolefin, polyvinylidene fluoride, polysulfone, polyether sulfone, and the like have a problem in that they are poor at trapping hydrophilic impurities, and therefore, in the case where a target to be removed is a hydrophilic substance, they have a low impurity removing property. On the other hand, polyacrylonitrile, cellulose acetate, and the like are relatively highly hydrophilic resins. However, they have problems in that they have a low membrane strength, and are susceptible to high temperatures, and to chemicals and thus they can be used only within a very small range of temperatures and pHs.
Under such circumstances, study has been conducted on a method for producing a porous membrane from a relatively-highly hydrophilic, highly-chemically-resistant polyamide resin. However, polyamide is only soluble in formic acid, concentrated sulfuric acid, and expensive fluorine-containing solvents, which are strong acids, at room temperature. Therefore, in the methods using NIPS, these solvents have been used. For example, the methods disclosed in Patent Documents 1 to 4 are membrane forming methods using formic acid as a solvent. However, these methods are problematic in safety and health. Furthermore, Patent Document 5 discloses a method of: casting a solution obtained by dissolving a mixture of polyamide 6 and polycaprolactone in hexafluoroisopropanol: and extracting caprolactone from the cast product to thereby making it porous. However, the solvent used and the polymer to be extracted and removed are both expensive, and this method is not practical.
On the other hand, a method using TIPS has also been studied. Non-patent Document 1 reportedly proves that it is possible to prepare a porous membrane from a system of polyamide 12 and polyethylene glycol. Furthermore, Patent Document 6 reportedly proves that it is possible to prepare a porous membrane from a system of polyamide 11 and ethylene carbonate, propylene carbonate, or sulfolane. Furthermore, Non-patent Document 2 describes that it is possible to prepare a porous membrane of polyamide 6 and polyamide 12 by using triethylene glycol as a solvent. However, all these methods just made it possible to form a porous membrane, and did not make it possible to process the membrane into a highly-water-permeable hollow fiber membrane and control the size of micropores.
In view of the circumstances, the present inventors have earnestly conducted a study on a polyamide hollow fiber membrane and a method for producing the polyamide hollow fiber membrane. As a result, the inventors have found that it is possible to prepare a polyamide hollow fiber membrane having excellent properties such as high hydrophilicity, water permeability, separating ability, strength, and the like, by forming the membrane by TIPS using a certain limited solvent as a solvent for membrane formation. This technique is disclosed in Patent Documents 7 and 8. However, according to the technique of Patent Documents 7 and 8, the pore size in the surface of the resulting polyamide hollow fiber membrane is as large as 100 μm. Therefore, the technique cannot satisfy the demand for a hollow fiber membrane with smaller pores, and needs further improvement.
Furthermore, most resins such as polyolefin usually contain a minute amount of catalyst that was used for polymerization, and contain additives such as a lubricant that was used for lubricating resin pellets when the pellets were inserted. Although these impurities are contained in minute amounts, there is a problem in that they may contaminate a separate liquid as an eluted material when the membrane is used as a hollow fiber membrane. Therefore, such a hollow fiber membrane has been sometimes difficult to use in semiconductor and pharmaceutical industries.